FIG. 1 diagrammatically illustrates the configuration of a conventional pulse width modulation (PWM) controlled DC-DC power converter. As shown therein a voltage terminal 10 is coupled to receive a reference voltage V1, such as that supplied by a digital-to-analog converter (DAC). This reference voltage is coupled to a reference terminal pin REF through a filter 11, such as that comprised of a resistor R4 and capacitor C3. The voltage terminal REF, in turn, is coupled to the non-inverting (+) input 21 of an operational amplifier (OP AMP) 20. The output 23 of the OP AMP is coupled to a PWM generator circuit 30, the output 33 of which is coupled through an inductor (L1) 40 to an output node 50.
The PWM generator circuit's output 33 is fed back as an output VOUT to the inverting (−) input 22 of amplifier 20 through a filter 60 containing a series resistor R2 coupled to a feedback path FB to the output 23 of the amplifier 20, and through a series connection of a capacitor Cl and a resistor R1 to a node COMP, that receives an amplified and filtered error signal from the amplifier 20. The PWM generator circuit 30 drives a load (represented by a resistor R3) coupled to the output node 50. The COMP terminal controls the PWM generator in a direction to minimize the difference between the voltage REF and the voltage VOUT.
PWM-based DC power supply integrated circuits are often required to have a user adjustable offset. Specifically, in some applications it may be desirable to have the output voltage generated at the output node VOUT different from the input reference voltage V1 by a prescribed (e.g., user-programmable) offset value. Depending upon the application, it may be necessary that this offset voltage be bi-directional, and must be repeatable and stable. Generating this offset should require a minimum number of external components, should not be dependent on other factors such as power supply voltage and, for economy of packaging, should require the least number of integrated circuit pins. In addition, to alleviate the design of supporting components, the offset should not require a stable current reference, but only a stable voltage reference (such as a bandgap voltage reference).